Drowning can easily be made to look like an accident, say if you are out at a lake, you slipped and fell into a strong current etc but you said a painless method.
So thats no good.
I suppose getting drunk, and I mean really drunk then going in the bath and you drowned. I got that idea from reading something on here today.
It would look like an accident but again, it depends on how you feel about the water part.
Maybe someone else on here can offer a different solution.